Which of these numbers is prime? ${39,\ 73,\ 74,\ 75,\ 93}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 39 are 1, 3, 13, and 39. The factors of 73 are 1 and 73. The factors of 74 are 1, 2, 37, and 74. The factors of 75 are 1, 3, 5, 15, 25, and 75. The factors of 93 are 1, 3, 31, and 93. Thus, 73 is a prime number.